1. Field of Invention
The present invention relates to a switching regulator and a control circuit and a control method therefor, in particular to such switching regulator which is capable of avoiding current reverse flow, and a control circuit and a control method therefor.
2. Description of Related Art
Switching regulators are often used in power supply circuits for converting an AC or a DC voltage to a regulated voltage. A switching regulator typically includes a control circuit and a power stage, wherein the control circuit controls the power stage by pulse width modulation (PWM) or pulse frequency modulation (PFM), to regulate the output voltage.
There are many types of switching regulators, such as buck switching regulator, boost switching regulator, inverter switching regulator, and buck-boost switching regulator, etc. The above regulators also can be categorized into two types according to their circuit structure: synchronous and asynchronous types. A synchronous switching regulator includes two power transistor switches (a synchronous buck-boost switching regulator includes four power transistor switches), while an asynchronous switching regulator replaces one of the power transistor switches by a diode.
FIG. 1A shows a prior art buck switching regulator. The switching regulator 10 includes an up-gate power transistor 11 connected between an input voltage Vin and a switching node 15, and a low-gate power transistor 12 connected between the switching node 15 and ground GND. When the up-gate power transistor 11 is turned ON and the low-gate power transistor 12 is turned OFF, current I1 flows from the input Vin, through the up-gate power transistor 11 and the inductor L 13, to charge the capacitor Cout 14 and generate an output voltage Vout. When the up-gate power transistor 11 is turned OFF and the low-gate power transistor 12 is turned ON, in the beginning current I2 flows from the ground GND, through the low-gate power transistor 12 and the inductor L 13, to the capacitor Cout 14; the current I2 decreases along with time. Referring to FIG. 1B, because the low-gate power transistor 12 is conductive bi-directionally, after the current I2 decreases to 0, a reverse current I3 flows from the capacitor Cout 14, through the inductor L 13 and the low-gate power transistor 12, to the ground GND. That is, the capacitor Cout 14 discharges and this is inefficient. The currents I1, I2 and I3 flow through the inductor L 13 and therefore they are referred to as the inductor current IL. In addition, to maintain the level of the output voltage Vout, the up-gate and low-gate power transistors 11 and 12 keep switching even in light load or no load conditions, which induces switching loss.
To improve the efficiency of the aforementioned synchronous switching regulator in light load or no load conditions, U.S. Pat. No. 6,580,258 proposes a control circuit and a method wherein the up-gate and low-gate power transistors are both turned OFF for a short period of time when the inductor current is lower then a threshold level. The proposed control circuit and method detects whether the inductor current is lower then a threshold level when the up-gate power transistor is OFF and the low-gate power transistor is ON. If yes, the low-gate power transistor is immediately turned OFF. However, because the low-gate power transistor is first turned ON and then turned OFF if the threshold condition is met, the low-gate power transistor has to frequently switch and this induces more switching loss. Besides, if the threshold level is not set properly, the reverse current might still occur.
In view of the above, the present invention proposes a switching regulator capable of avoiding current reverse flow and reducing the switching operation of the power transistors, and a control circuit and a control method therefor.